This application and Ser. No. 09/240,150 are copending reissue applications of the same original patent. 
The present invention concerns improvements in catalysts and in catalytic processes. More especially it concerns catalysts and processes for dehydrogenation of alkanes.
It is known to dehydrogenate isobutane to isobutene using direct dehydrogenation at low space velocity (GHSV=100-1000 hrxe2x88x921). The conventional industrial process has several inherent disadvantages:
(a) it is an endothermic reaction, requiring high thermal input;
(b) the yield of isobutene is equilibrium limited; and
(c) at temperatures favouring high yields of isobutene, the rate of catalyst de-activation is also high.
Improvements to the conventional process have included the addition of either steam (eg U.S. Pat. No. 4,926,005 and 4,788,371) or hydrogen (eg U.S. Pat. No. 4,032,589) to the gas feed. The function of the hydrogen is as a diluent, and to reduce the deposition of carbon on the catalyst. The steam improves thermal conduction through the catalyst bed and reduces the deposition of carbon on the catalyst, and hence it too has been used as a diluent. The catalysts used in industry include platinum on alumina, platinum on tin oxide and chromium oxide-based catalysts. There remains a need for an improved process for the dehydrogenation of alkanes, especially for the dehydrogenation of isobutane, which is a starting material for MTBE (methyl-tert-butyl-ether) production. The conventional processes require high inputs of energy and the capital cost of a catalytic reactor designed to supply large amounts of heat is particularly high. Moreover, conventional processes demonstrate rapid catalyst deactivation, so that expensive and complex catalyst regeneration has to be designed into the equipment and the process.
The present invention provides an improved process and novel catalyst for alkane dehydrogenation.
Accordingly, the invention provides a process for the dehydrogenation of an alkane to form an alkene, comprising passing a feedstock comprising said alkane in the gas phase in admixture with oxygen and in the absence of added steam over a dehydration and oxidation catalyst comprising a platinum group metal deposited upon a support.
The invention provides also a catalyst for alkane dehydrogenation, comprising platinum deposited upon a support which is a mixture of tin oxide and zirconium oxide. The invention also provides a process for the dehydrogenation of an alkane to form an alkene, comprising passing a feedstock comprising said alkane in the gas phase over this catalyst.
The present processes and catalyst are advantageous over the known processes and catalyst by reason of one or more of such features as higher yield of the alkene, higher selectivity to the alkene, lower operating temperature, lower heat input, a simpler system and lower catalyst deactivation.
There is much prior an  art on the dehydrogenation of alkanes to alkenes, (though a scant amount on the oxidative dehydrogenation of alkanes to alkenes), yet the present improvements were not realised before. As explained in the U.S. specification 4,788,371 mentioned above, the dehydrogenation of hydrocarbons is endothermic. In a system employing a dehydrogenation catalyst only, it is typically necessary to add superheated steam at various points in the process or to intermittently remove and reheat the reaction stream between catalyst beds. In an improvement, processes were developed which utilised a two-catalyst system with distinct beds or reactors of dehydrogenation or selective oxidation catalysts. The purpose of the selective oxidation catalysts was to selectively oxidise the hydrogen produced as a result of the dehydrogenation with oxygen that had been added to the oxidation zone to generate heat internally in the process. The heat generated would typically be sufficient to cause the reaction mixture to reach desired dehydrogenation temperatures for the next dehydrogenation step. The US specification explains that in its invention one specific catalyst can be used to accomplish both the dehydrogenation and oxidation reactions. It discloses a process for the steam dehydrogenation of a dehydrogenatable hydrocarbon with oxidative reheating which comprises contacting a dehydrogenatable hydrocarbon comprising C2-C15 paraffins and steam at a steam to hydrocarbon molar ratio of from 0.1:1 to 40:1, at a pressure from 0.1 to 10 atmospheres, a temperature of from 400xc2x0 to 900xc2x0 C., and a liquid hourly space velocity of from 0.1 to 100 hrxe2x88x921 with a catalyst in the first reaction zone of a reactor containing a plurality of reaction zones and introducing an oxygen-containing gas into the second, and every other reaction zone of the plurality of reaction zones such that the total rate of the oxygen-containing gas introduced into the reaction zone ranges from 0.01 to 2 moles of oxygen per mole of C2-C15 paraffin feed wherein the catalyst is comprised of from 0.1 to 5 weight % platinum, and from 0.01 to 5 weight% potassium or cesium or mixtures thereof on an alumina support having a surface area of from 5 to 120 m2/g and recovering the products of the reaction. Though the specification mentions the possibility of a single reaction zone within a single reactor with single inlet and outlet parts, all co-feeds entering the inlet of the reactor and products and by-products leaving the system through the reactor outlet part, there is no Example illustrating this concept. Moreover, the present broad process involving passing an alkane in admixture with oxygen over a dehydrogenation and oxidation catalyst is not a steam dehydrogenation; instead, it is carried out in the absence of added steam (though some steam is formed by reaction of the oxygen with hydrogen which is present).
FIG. 3 shows a schematic diagram of a reactor 10 for performing this process. As shown in FIG. 3, the present broad process involves passing an alkane stream 10 in admixture with oxygen over the dehydrogenation and oxidation catalyst in the reactor 20 to result in a product stream 30. In this aspect of the present invention, we have discovered that oxygen in the absence of added steam is advantageously admixed with the alkane and passed over the catalyst, so that heat produced by the exothermic reaction of the oxygen with hydrogen which is present provides, partially or fully, the heat required by the endothermic dehydrogenation. The hydrogen required for the reaction with the oxygen can be introduced into the reaction zone, but this is not preferred. Advantageously, the hydrogen is hydrogen produced by the dehydrogenation of the alkane to alkene, so as to shift the equilibrium in favour of the alkene. Preferably, the amount of oxygen is such that the dehydrogenation is carded  carried out under adiabatic conditions, so that no heat is supplied (or removed) from the reaction. Especially preferred is the amount of oxygen being such that the endothermic dehydrogenation is balanced by the exothermic reaction of the oxygen with hydrogen which is present so that the temperature remains constant (this situation is referred to herein as thermally neutral conditions). Thus, the optimum temperature for yield, life of catalyst etc can be maintained, eg so that at least 95% selectivity to the alkene is obtained.
The amount of oxygen is desirably less than the amount of the alkane, on a molar basis, and preferably less than half the amount of the alkane on this basis. For example, employing isobutane as the alkane, it is preferred that the amount of oxygen be below that indicated by the stoichiometry of the reaction equation:
C4H10+0.5O2xe2x86x92C4H8+H2O. 
The optimum amount of oxygen will vary with the desired operating temperature, and as a guide we would predict that the maximum amount of oxygen for highly selective, thermally neutral, dehydrogenation of isobutane be 5% at 450xc2x0 C., 7.5% at 500xc2x0 C. and 9% at 550xc2x0 C., based on the combined volumes of isobutane and oxygen.
The present oxidative dehydrogenation is usually carried out at a temperature from 350xc2x0 to 550xc2x0 C., for instance at a temperature from 350xc2x0 to 480xc2x0 C., for example when the platinum group metal comprises platinum and the support comprises alumina.
The oxidative dehydrogenation is preferably carried out under relatively high space velocities, such as an alkane, and especially a total, gas hourly space velocity (GHSV) of 1000 to 5000 hrxe2x88x921, for example for isobutane.
The operating pressure is conveniently atmospheric, but the dehydrogenation can be operated at above or below atmospheric pressure. If desired, diluent gases can be used, although hydrogen is not recommended as explained above; in addition, it would be an added process cost.
The alkane which is dehydrogenated is preferably raw material, not alkane which has already been partially dehydrogenated.
The oxygen can be employed as such but conveniently it is employed as a component of an oxygen-containing gas, particularly air.
The platinum group metal dehydrogenation and oxidation catalyst can be such a catalyst known in the art. The platinum group metal (ruthenium, rhodium, palladium, osmium, iridium and platinum) is preferably platinum. The catalyst preferably contains 0.1 to 3% by weight of the platinum group metal, eg platinum. The support can be for example alumina, silica, magnesia, titania, zirconia or a mixture or joint oxide (eg an alumina silicate) thereof, or a Group IIA or IIB (eg zinc) aluminate spinel. Groups IIA and lIB are as given in the inside front cover of the CRC Handbook of Chemistry and Physics, 60th edition, CRC Press, 1980. Commonly, the support comprises (ie consists of or includes) alumina. For instance, the catalyst contains as support 10-99.9% by weight of alumina. Promoters can be employed with the platinum group metal. Preferred as promoter is tin oxide. The promoter, when present, is usually employed as 0.1-5% by weight of the catalyst. The catalyst can be obtained in conventional ways, for example by impregnating the support with a precursor of the platinum group metal and a precursor of any co-promoter, and calcining.
A particularly advantageous catalyst for the present oxidative dehydrogenation of an alkane in the absence of added steam, though it can be used advantageously for the oxidative dehydrogenation in the presence of added stem  steam, and indeed for the direct dehydrogenation, is a novel catalyst. This catalyst for alkane dehydrogenation comprises platinum deposited upon a support which is a mixture of tin oxide and zirconium oxide. The catalyst contains a catalytically effective amount of the platinum. Usually the catalyst contains 0.1 to 3% by weight of platinum, calculated as metal. Additional catalytically active components can be present, though preferably the catalytically active component consists essentially of platinum. The catalyst contains a supporting amount of the mixture of tin oxide and zirconium oxide. Additional support components can be present. The common support component alumina, however, has been found to be disadvantageous. Preferably, therefore, the catalyst contains substantially no alumina. It is preferred that the support consists essentially of the mixture of tin oxide and zirconium oxide. Usually the catalyst contains 6-60, preferably 10-60, especially 15-30, % by weight of the tin oxide (measured as tin oxide). Usually the catalyst contains 37-94.9, preferably 70-85, % by weight of the zirconium oxide. The weight ratio of the tin oxide to the zirconium oxide is preferably 1:3-9, especially 1:3-5. In a preferred embodiment, the catalyst comprises 0.1 to 3% by weight of platinum, calculated as metal, 10 to 60% by weight of tin oxide, the balance being zirconium oxide. A particular catalyst has a support comprising SnO2 and ZrO2 in a weight ratio of approximately 1:4. One preferred embodiment of the catalyst of the invention is prepared by impregnating 1% (by weight, calculated as metal) of a platinum salt or compound onto a co-precipitate of SnO2 and ZrO2 in a weight ratio of 1:4.
The catalyst of the invention may comprise in addition other components such as promoters and/or stabilisers. The catalyst may be in the form of pellets or other shapes, for example produced by pelletisation or extrusion, or may be supported on high surface area monoliths such as ceramic or metal honeycomb monoliths.
The mixture of SnO2 and ZrO2 may be formed in a variety of ways and there may be a chemical interaction or compound formation between the components which is as yet not fully understood. The preferred method of preparation is by co-precipitation; suitably by adding NaOH to a mixture of tin and zirconium salts in aqueous solution. The mixture may then be dried and calcined, especially to yield a powdered material with moderately high surface area (typically 95 m2gxe2x88x921) and narrow pore-size distribution (most of the pores having a radius of about 2 nm), before impregnation with an aqueous solution of a platinum salt. The impregnated catalyst is suitably dried and calcined again.
The invention further provides a process for the dehydrogenation of alkanes to form alkenes, comprising passing a feedstock comprising said alkane in the gas phase over a catalyst according to the invention. Advantages of the present catalyst and process are indicated in the Examples hereafter. In particular, the invention provides the use of the catalyst in the oxidative dehydrogenation of an alkane, whereby extended durability before regeneration is achieved.
The process employing the novel catalyst is particularly advantageous when operated as an oxidative dehydrogenation reaction. That is, the invention includes a process for the oxidative dehydrogenation of alkanes to form alkenes, comprising passing a feedstock comprising said alkane in the gas phase in admixture with oxygen over a catalyst according to the invention. The oxygen can be employed as such, but conveniently it is employed as a component of an oxygen-containing gas, particularly air.
The oxidative dehydrogenation using the novel catalyst can be carried out mutatis mutandis as described above for oxidative dehydrogenation in the absence of added steam using catalysts in general. For instance, the oxidalive dehydrogenation, with or without added steam, using the novel catalyst is preferably carded out under relatively high space velocities, such as an alkane, and preferably a total, GHSV of 1000 to 5000 hrxe2x88x921, for example for isobutane.
Preferably, the oxidative dehydrogenation using the novel catalyst is operated under adiabatic conditions, especially thermally neutral conditions. The amount of free oxygen in the feedstock is preferably, therefore, controlled to achieve this under the other operating conditions chosen. In particular, the amount of oxygen required increases with increasing temperature. It has been found that operation under adiabatic conditions offers the opportunity to overcome many of the disadvantages of direct dehydrogenation. In preferred embodiments, the process of the invention:
i) provides heat within the catalyst bed by reacting exothermically with some of the hydrogen being formed;
ii) by consuming hydrogen, can shift the equilibrium in favour of the desired products; and
iii) suppresses two of the major causes of catalyst deactivation, that is over-reduction of the catalyst and carbon deposition.
The concentration of oxygen should be carefully controlled at adiabatic conditions, and it is believed that the amount of oxygen should be maintained below stoichiometric relative to the amount of hydrogen present. There are two primary reasons for requiring that the amount of oxygen be carefully controlled, firstly to avoid unwanted products being produced, either from partial or deep oxidation, and secondly to prevent excessive temperature excursions caused by large exotherms.
Desirably, the oxidative dehydrogenation reaction using the novel catalyst is carried out at a temperature of from 350xc2x0 to 550xc2x0 C., more preferably in the range 400xc2x0 to 530xc2x0 C., especially 440xc2x0 to 510xc2x0 C. The operating pressure is conveniently atmospheric, but the process may be operated at above or below atmospheric pressure. If desired, diluent gases may be used, although hydrogen is not recommended since it would be consumed and be an added process cost.
Although the present invention, whether involving the novel catalyst or not, is described herein with particular reference to the oxidative dehydrogenation of isobutane, the invention should not be considered as limited thereto, and may be applied to alkanes in general, and the novel catalyst may also find application in direct dehydrogenation. Nonetheless, it is believed that the greatest benefits arise in oxidative dehydrogenation. The alkane is usually of 2-15, preferably 2-5, particularly 3 or 4, carbon atoms. The alkane can be linear, though preferably it is branched.